This invention relates to a proximity sensor assembly or module for detecting the passing of a ferromagnetic article such as a gear tooth, and more particularly to such a sensor comprising an assembly of a magnet structure and an integrated circuit chip having a Hall element, the chip being mounted at one end of the magnet structure and in the magnetic field created by the magnet structure, and a container. The Hall element generates an electrical signal related to the strength of the magnetic field normal to the plane of the Hall element. As a ferromagnetic article approaches the Hall element, the strength of the magnetic field normal to the Hall element is changed. Thus the distance between the article and the Hall element is reflected in the electrical signal generated by the Hall element. This allows the Hall-effect sensor to sense the distance between the sensor and ferromagnetic object.
A very appropriate application for a sensor of this type is in the measurement of rotational speed or rotational position of a gear or gear-shaped disk. By placing this type of sensor adjacent the periphery of the gear, the proximity, passing, and rate-of-passing of the gear teeth by the sensor is reflected in the electrical signal. Thus, the sensor can be used as a speedometer, tachometer, or for monitoring rotary or linear motion or position.
The typical Hall-effect sensor employs a magnet structure consisting of a cylindrical permanent magnet with a flat end. The Hall chip is positioned adjacent the flat end and with the plane of the Hall element parallel to the plane of the magnet end.
The integrated circuit chips of such typical proximity sensors almost always include an essentially linear Hall-voltage amplifier for amplifying the Hall output voltage. Also included in many such Hall integrated circuits is a Schmitt trigger circuit for producing a binary output signal that changes from one level (a standby level) to the other binary level (an action level) when a ferrous article approaches to within a critical distance at which the magnetic field, normal to a major face of the chip, exceeds a predetermined magnitude. These circuits are normally DC connected so that the sensor is capable of sensing passing ferrous articles at zero rate (e.g. one a year) up to a high rate (e.g. 100 KHz).
In the past, proximity sensor units tended to be developed as an application-specific unit with a casing that allows the unit to be mounted and positioned at the sensing location, with a plug-type connector to allow the unit to communicate through wire to a remote signal processing system, and with application-specific circuitry within the casing for processing the sensor signal so that it is suitable for transfer through the wire to the remote equipment.
Most of the aspects of this overall unit (mounting, signal-processing, plugging) are quite fault tolerant and the accuracy and environmental sensitivity requirements of these aspects can be easily satisfied using normal design and manufacturing tolerances and approaches. Unfortunately, this is not true of the sensor aspect or section of the unit. The physical relationship and stability of the Hall sensor components (the Hall element and chip, the leads from the chip, the magnet, and the directly associated components) to one another and appropriate protection of the sensor elements from environmental damage are critical to the accuracy and usefulness of the sensor signal.
This creates a serious design and manufacturing problem for the units. If the whole unit is designed and manufactured using normal standards, the sensor aspect is often not reliable. If, on the other hand, the unit is designed and manufactured to the standard required by the sensor section, the cost of the entire unit is often unacceptably high. Efforts to impose different standards on the sensor section versus the other sections adds to the costs and can be impractical. Furthermore the cost of designing a high quality sensor section for each new application situation imposes unacceptable high costs on the project and frequently fails to allow accumulation of data on the long-term characteristics of a particular design to allow accurate long-term reliability forecasts. Lack of accurate long-term reliability forecasts can create unexpected product failures, possible product liability, and expensive product recall requirements.
These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.
It is, therefore, an outstanding object of the present invention to provide a gear tooth sensor module designed to allow flexible use of the module as a subassembly in a wide range of equipment and situations.
Another object of this invention is to provide a gear tooth sensor module which is itself a rugged and stable unit capable of enduring wide-ranging environmental conditions without deterioration.
A further object of the present invention is to provide a gear tooth sensor module that can be provided as a basic magnet holder, with or without printed circuit board attached, or as a finished over-molded module with or without proper application (e.g., automotive) connector attached, either using a shell and O-rings or by direct over-molding of the sensor module.
It is another object of the invention is to provide a gear tooth sensor module which can be assembled of preformed parts and in a simple manner which is conducive to reliable automated assembly.
A still further object of the invention is to provide a gear tooth sensor module with internal lead jigging and lead and component short-circuit prevention.
It is a further object of the invention to provide a gear tooth sensor module with lead separation and directional selection at the exit of the leads from the module.
It is a further object of the invention to provide a gear tooth sensor module which provides a positive bias of the sensor chip toward the magnet in order to provide positional stability and therefore signal stability over a wide range of thermal cycles.
It is a further object of the invention to provide a gear tooth sensor module which allows different magnet materials and sizes, different lead lengths on the sensor package, various PC board designs (through hole or surface mount), and vertical or horizontal mounting.
It is a further object of the invention to provide a gear tooth sensor module which allows inclusion of a PC board and associated electronics, which reduces the number of connections between the module and the application connector and thereby provides maximum reliability.
It is a further object of the invention to provide a gear tooth sensor module which is capable of being manufactured of high quality and at a low cost, and which is capable of providing a long and useful life with a minimum of maintenance.
With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto, it being understood that changes in the precise embodiment of the invention herein disclosed may be made within the scope of what is claimed without departing from the spirit of the invention.